1. Field of the Invention
The present invention relates to a liquid crystal display device, a method for producing the same, and a projection-type display apparatus including the liquid crystal display device.
2. Description of the Related Art
Recently, liquid crystal display devices for displaying images using polymer dispersed liquid crystal (PDLC) have been developed. PDLC is described in, for example, LIQUID CRYSTALS, APPLICATIONS AND USES, Vol. 1, Chapter 14, edited by Brendra Bahadur, World Scientific Publishing, 1990. A liquid crystal display device for displaying images using PLDC will be referred to as a "PDLCD" device.
A PDLC material has a structure including many liquid crystal droplets dispersed in a polymer matrix. When no voltage is applied to the PDLC material, liquid crystal molecules in the liquid crystal droplets are in a random orientation state. In this state, light incident on the PDLC material is scattered. When a voltage is applied to the PDLC material, the liquid crystal molecules in the liquid crystal droplets are aligned in the direction of the electric field. In the case where the ordinary light refractive index no of the liquid crystal molecules is substantially equal to the refractive index n.sub.p of the polymer matrix, the light incident on the PDLC material is transmitted without scattering. Thus, the light-scattering state or light-transmitting state of the PDLC material can be selected by controlling the voltage applied. PDLCD devices display images using such an optical phenomenon of the PDLC material.
A PDLCD device and a projection-type display apparatus including a PDLCD device are disclosed in, for example, Japanese Laid-Open Patent Publication No. 3-58021. The PDLCD device disclosed in this publication displays images having a high contrast ratio using an active element and a driving circuit usable for a conventional TN-mode liquid crystal display device. Such a PDLCD device is designed so that the average diameter R (.mu.m) of the liquid crystal droplets, the cell thickness d (.mu.m), the refractive index anisotropy (birefringence) .DELTA.n of the PDLC material, and the maximum possible voltage V (Vmax) which can be applied to the PDLC material fulfill all the conditions represented by the following expressions. The average diameter R of the liquid crystal droplets and the cell thickness d influence the scattering characteristic and also the threshold characteristic (indicated by the threshold voltage) of the PDLC material. EQU 0.3&lt;R.multidot..DELTA.n&lt;0.7 EQU 4R&lt;d&lt;8R EQU 0.5.multidot.R.multidot.Vmax&lt;d&lt;R.multidot.Vmax
Projection-type color display apparatuses for displaying color images have also been proposed. Such a projection-type color display apparatus includes three PDLC panels for receiving red (R), green (G) and blue (B) components of light. The three components of the light are combined to obtain a color image. However, the scattering characteristic of a PDLC material depends on the wavelength of light. The scattering intensity of a PDLC material with respect to red light having a relatively long wavelength is relatively low. Due to such dependence, the three PLDC panels used in combination in one display apparatus each need to have an optimum structure for the respective color of light. For example, the average diameter of the liquid crystal droplets and the cell thickness need to be adjusted fcr each panel.
In a color display apparatus disclosed in, for example, Japanese Laid-Open Patent Publication No. 3-58022, the average size of the liquid crystal droplets and the cell thickness are adjusted to be optimum for the respective color. In particular, this publication indicates the following conditions. EQU 0.3&lt;R.sub.0 .multidot..DELTA.n&lt;0.7 EQU 4R.sub.0 &lt;d.sub.0 &lt;8R.sub.0 EQU 0.9R.sub.0 /.lambda..sub.0 &lt;R.sub.x /.lambda..sub.x &lt;1.1R.sub.0 /.lambda..sub.0 EQU 0.9d.sub.0 /.lambda..sub.0 &lt;d.sub.x /.lambda..sub.x &lt;1.1d.sub.0 /.lambda..sub.0
where .DELTA.n represents the birefringence of the PDLC material, R.sub.x (.mu.m) represents the average diameter of the liquid crystal droplets in each PDLC panel for receiving each color of light, d.sub.x (.mu.m) represents the cell thickness of each PDLC panel, .lambda..sub.x represents the dominant wavelength of each color of light, R.sub.0 (.mu.m) represents the average diameter of the liquid crystal droplets in the PDLC panel for receiving green light (dominant wavelength .lambda..sub.0 =540 nm), and d.sub.0 represents the cell thickness of the PDLC panel for receiving the green light.
In a color display apparatus disclosed in, for example, Japanese Laid-Open Patent Publication No. 3-98022, the average diameter of the liquid crystal droplets is equal among the three PDLC panels but the cell thicknesses thereof are optimized for the respective colors. In particular, this publication indicates the following conditions. EQU 0.3&lt;R.multidot..DELTA.n&lt;0.7 EQU 4R&lt;d&lt;8R EQU 0.9d.sub.0 /.lambda..sub.0.sup.1/2 &lt;d.sub.x /.lambda..sub.x.sup.1/2 &lt;1.1d.sub.0 /.lambda..sub.0.sup.1/2
where R (.mu.m) represents the average diameter of the liquid crystal droplets, and the other symbols represent the same parameters as in the conditions mentioned above in connection with Japanese Laid-Open Patent Publication No. 3-58022.
Conventionally, as described above, the structure of the PDLC panel used in a PDLCD device needs to be optimized in consideration of the threshold voltage and the scattering characteristic of the PLDC material. In a color display apparatus using a PDLC material, the scattering characteristic of each PDLC panel needs to be optimized for the respective color of light. These requirements decrease the productivity of the display devices and apparatuses, and increase the production cost.